Effects of oxygen concentration and body weight on maximum feed intake, growth and hematological parameters of Nile tilapia, Oreochromis niloticus

Feed intake and satiation in fish are regulated by a number of factors, of which dissolved oxygen concentration (DO) is important. Since fish take up oxygen through the limited gill surface area, all processes that need energy, including food processing, depend on their maximum oxygen uptake capacity. Maximum oxygen uptake capacity relative to body weight in bigger fish is smaller than in smaller fish because the gill surface area is allometrically related to body weight. In this study, effects of DO concentration and body weight on maximum feed intake, growth and hematological parameters of Nile tilapia (Oreochromis niloticus) were investigated. Two weight classes of fish (21 g and 147 g) were used. For each class, six tanks were employed of which half were exposed to one of two DO levels (about 3.0 mg/L and 5.6 mg/L). Fish were fed to apparent satiation twice per day with a commercial diet. The results showed that (1) feed intake and growth of the fish at high DO level were significantly higher than at low DO level (P < 0.01), (2) relative feed intake and growth of small fish were significantly higher than of big fish (P < 0.001), and (3) fish at low DO level made no hematological adjustments (P ≥ 0.5). Data suggest that (1) the limitation of the gill surface area results in lower feed intake and growth of fish at low DO concentration than at high DO concentration and (2) the allometric relationship between the gill surface area and body weight results in lower relative feed intake, which in turn results in lower relative growth in big fish than in small fish.